An electric storage apparatus formed of a lithium-ion battery is known as a driving or auxiliary power source for an electric car, a hybrid car and the like. The lithium-ion battery experiences a reduced battery capacity and thus suffers deterioration due to repeated charge and discharge. Since the deterioration rate of the lithium-ion battery follows the Arrhenius law, the deterioration rate is increased as the temperature rises.
Patent Document 1 (Japanese Patent Laid-Open No. 2010-060408) has disclosed a method of determining the deterioration of a lithium-ion battery in which the deterioration state of the lithium-ion battery is determined on the basis of information about changes in voltage of the lithium-ion battery obtained in a diagnostic mode wherein the lithium-ion battery is successively discharged and charged at a fixed power value.